Most of conventional class D audio amplifiers suffer from the transient noise, such as “pop” or “click”. The noise is usually caused by charging or discharging an input capacitor, an output capacitor or a bootstrap capacitor of the class D audio amplifier. FIG. 1 schematically shows a prior art class D audio amplifier. In FIG. 1, an input signal Vinput is provided to an input terminal NIN of an audio control circuit via an input capacitor Cin and an input resistor Rin. A voltage reference of 0.5 VDD is provided to another input terminal PIN of the audio control circuit, wherein VDD is the power supply voltage of the class D audio amplifier. An integrating capacitor CINT is coupled between the two input terminals of the audio control circuit. An output signal of the class D audio amplifier is provided to a speaker via a low pass filter comprising an inductor Lf and a capacitor Cf. In FIG. 1, Cout represents the output capacitor and Cbs represents the bootstrap capacitor.
In FIG. 1, when the system starts up, the input capacitor Cin and the output capacitor Cout need to be charged to half of the power supply voltage in order to prevent output signal being clipped in both positive and negative side. That means Vsw=VNIN=VPIN=0.5 VDD. FIG. 2A shows the waveform of the current Ispeaker flowing through the speaker when the input capacitor Cin and the output capacitor Cout are charged. The pulse of the current Ispeaker is caused by a step of the charging current of the output capacitor Cout. Inevitably, the pulse of the current Ispeaker includes some components which are audible to humans, i.e., the frequency of some current components are within the range of 20 Hz-20 kHz, and will bring in noise in the speaker.
In FIG. 1, the charging current paths of the input capacitor Cin, the output capacitor Cout and the bootstrap capacitor Cbs are indicated by broken lines with arrows. As can be seen from FIG. 1, because of the resistor Rf, a voltage across the output capacitor Cout will be larger than 0.5 VDD when the voltage VNIN at the input terminal NIN reaches 0.5 VDD. Then, the output capacitor Cout will be discharged during the normal operation of the class D audio amplifier. The discharging current causes “pop” or “click” noise in the speaker. FIG. 2B shows the waveform of the current Ispeaker and the voltage Vspeaker of the speaker during the normal operation of the circuit in FIG. 1. As can be seen from FIG. 2B, there are fluctuations in the current Ispeaker and the voltage Vspeaker in interval t0-t1 during when the output capacitor Cout is discharged. The fluctuations in the current Ispeaker and the voltage Vspeaker cause noise.
The present invention pertains to provide a class D audio amplifier with noise suppression.